The Folk-Nature of Sacred Stones
By Gary R Varner
"...Thunderstones are stones reportedly found only where lightning strikes, are black in color with white streaks running through them. They also have great power for healing and strength. They are effective in the treatment of jaundice, lameness, cataracts, convulsions, consumption, goiter, snakebite and childbirth and, if carried on the person, relieve rheumatism...." [Click here] for a larger excerpt.
FFC always says it is special when a block rock with a white stripe is found as the wedge in a split-wedged rock. The book excerpt also mentions rocks guiding people to the underworld.